The invention relates to a locking mechanism for a spindle drive.
Locking devices as previously known employ limiting switches to monitor and control the travel of the moving parts of the spindle drive. If these devices fail or are bypassed, spindle travel is stopped by torque switches. Due to the inertia of the system comprising the drive motor and gearing, the actual moment applied to the components of the member connected to the spindle, typically a valve, will exceed the magnitude of the moment set at the torque switch. The moment generated can be of such a magnitude that damage will occur to the components of the valve. Due to variations in the friction generated within the drive as well as between the spindle nut and the spindle, the torque indicated by the torque switches will not be an accurate measure of the axial forces exerted on the spindle and overstressing may occur.
In addition to the load generated by the torque switches, thermal stresses caused by differences in tension due to a temperature gradient in the spindle when lodged in an end position will be present. Such stresses can result in bending of the spindle when subjected to an axial load in addition to sizing or freezing of a sliding valve connected to the spindle. Jamming of the valve can lead to costly operational shut-downs or curtailments in service and it is therefore necessary to employ devices of maximum operational reliability. This is especially true in the case of nuclear power plants, where there is only a very limited amount of time available for maintenance due to the dange of radiation.
It is therefore an object of the present invention to provide a locking mechanism in which locking elements are used to avoid any overloading of the valve spindle and its driving components and thus minimize any damage or jamming of the valve.
The locking mechanism of the present invention is provided with locking elements for the spindle drive. A drive unit selectively provides for motorized or manual drive of a gearing which axially adjusts the spindle position. Travel of the spindle is stopped by the locking elements and the gearing has a friction clutch located between the drive unit and the gearing to break overtravel of the spindle. Switching devices actuated by the gearing regulate travel of the valve spindle. The gearing which axially adjusts the valve spindle has a gear body which is rigidly connected with a spindle nut and carries two locking elements, the elements being operative in opposite directions of rotation. Two arms which are resiliently mounted to deflect along the longitudinal axis of the spindle but rigid in the horizontal direction are provided on a flange of the housing. Each arm carries one locking ring which interacts with a respective locking element on the gear body and deflects resiliently relative to its respective arm. The gear body is engaged between the arms by means of a bearing and deflects along the axis of the valve spindle. Two electric switches, each being actuated by the deflection of the associated arm mounted on the flange, and one terminal switch which provides for a periodic bypass of the lower switch and is actuated by the valve lift operate to regulate the travel of the spindle. Additionally, the gearing is provided with a cam device to shift the upper locking element relative to its respective resiliently mounted arm.